This paper evaluates a smoking prevention intervention aimed at vocational school students, consisting of an existing Dutch in-school program (three lessons each lasting 50 min) and a computer-based tailored out-of-school program (three tailored letters with smoking prevention messages mailed to students' homes). Nineteen schools that already participated in the in-school program were randomly assigned to the in-school or to the combined in-school and out-of-school condition. The remaining 17 schools were randomly assigned to the out-of-school condition or to the control group. Effect outcomes were assessed at 6, 12 and 18 months after a pre-test, and were based on initiation among never-smokers and continuation among ever-smokers. Twelve months after the pre-test (post-test 2), the in-school intervention was successful in preventing vocational school students from continuing to smoke, compared with students in the control condition [odds ratio (OR) = 0.49; 95% confidence interval (CI) = 0.29-0.84]. Eighteen months after the pre-test (post-test 3), the tailored out-of-school intervention was successful in preventing smoking initiation, compared with students in the control condition (OR = 0.42; 95% CI = 0.18-0.96). The effect of the combined approach was not larger than the sum of the effects of the in-school and the out-of-school effects.